Semiconductor Device Including a LDMOS Transistor and Method

ABSTRACT

In an embodiment, a semiconductor device includes a semiconductor substrate having a front surface, a LDMOS transistor in the front surface, and a metallization structure arranged on the front surface. The metallization structure includes at least one cavity arranged in at least one dielectric layer.

BACKGROUND

There is an ongoing need for solid state circuits adapted to operate athigher and higher frequencies, including microwave frequencies. As usedherein, the term “microwave” is intended to refer to frequencies at orabove about 200 mega-Hertz, for example in the range of 300 Mhz to 3GHz. Various transistor structures have been created that are capable ofproviding gain in such frequency ranges. A LDMOS (Lateral Diffused MetalOxide Semiconductor) transistor is an example of such a transistorstructure.

At high frequencies parasitic coupling between conductive portions ofthe metallization structure of transistor structures can limitperformance. One way of decreasing parasitic coupling is through the useof dielectric material with a lower dielectric constant within one ormore layers of the metallization structure. Such dielectric materialsmay be called low-k dielectrics and typically have a dielectric constantk in the range of 2.5 to 4.1. However, such low-k dielectric materialsmay be mechanically sensitive and more difficult to process.

Therefore, further improvements to reduce parasitic coupling insemiconductor devices for use at higher frequencies are desirable.

SUMMARY

In an embodiment, a semiconductor device includes a semiconductorsubstrate having a front surface, a LDMOS transistor the front surfaceand a metallization structure arranged on the front surface. Themetallization structure includes at least, one cavity arranged in atleast one dielectric layer.

In an embodiment, a LDMOS transistor includes a semiconductor substrateincluding a front surface, a doped source region, a gate, a doped drainregion, a doped channel region, a doped drain drift region and a dopedbody contact region arranged in the front surface, a metallizationstructure arranged on the front surface, the metallization structureincluding one or more dielectric layers, one or more conductiveredistribution layers arranged between one or more dielectric layers andone or more conductive vias extending through one or more dielectriclayers, and at least one cavity defined by dielectric material, thecavity having a dielectric constant which is lower than a dielectricconstant of the dielectric material defining the cavity.

In an embodiment, a method includes applying a dielectric layer to afront surface of a semiconductor substrate including a LDMOS transistorstructure, forming an opening in at least one dielectric layer andcovering the opening with a further dielectric layer to seal the cavity.

In an embodiment, a method includes positioning one or more cavitieshaving a dielectric constant that is less than a dielectric constant ofdielectric material of a metallization structure arranged on a LDMOStransistor in positions so as to reduce one or more of the groupconsisting of the electric field in the metallization structure at thosepositions, capacitive coupling between drain and gate, capacitivecoupling between source and gate, and capacitive coupling between sourceand drain.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description, and uponviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The elements of the drawings are not necessarily to scale relative toeach other. Like reference numerals designate corresponding similarparts. The features of the various illustrated embodiments can becombined unless they exclude each other. Exemplary embodiments aredepicted in the drawings and are detailed in the description whichfollows.

FIG. 1a illustrates a semiconductor device including a LDMOS transistorand a metallization structure.

FIG. 1b illustrates a semiconductor device including a

LDMOS transistor and a metallization structure.

FIG. 2a illustrates a cavity formed in a dielectric layer of ametallization structure.

FIG. 2b illustrates a cavity formed in a dielectric layer of ametallization structure.

FIG. 2c illustrates a cavity formed in a dielectric layer of ametallization structure.

FIG. 3a illustrates a cross-sectional view of a semiconductor deviceincluding a LDMOS transistor and a metallization structure.

FIG. 3b illustrates a plan view of the semiconductor device of FIG. 3 a.

FIG. 4 illustrates a semiconductor device including a LDMOS transistorand a multilevel metallization structure.

FIG. 5a illustrates a flow chart of a method for a cavity in ametallization structure of a LDMOS transistor.

FIG. 5b illustrates a flow chart of a method for a cavity in ametallization structure of a LDMOS transistor.

FIG. 6 illustrates a semiconductor device including a LDMOS transistorand a first level of a multilevel metallization structure.

FIG. 7 illustrates a semiconductor device including a LDMOS transistorand a first level of a multilevel metallization structure.

FIG. 8 illustrates a semiconductor device including a LDMOS transistorand a cavity within a first level of a multilevel metallizationstructure.

FIG. 9 illustrates a semiconductor device including a LDMOS transistorand a first level of a multilevel metallization structure with a drainrunner.

FIG. 10 illustrates a plan view of the first level of the multilevelmetallization structure of FIG. 9.

FIG. 11 illustrates a semiconductor device including a LDMOS transistorand a second level of a multilevel metallization structure.

FIG. 12 illustrates a semiconductor device including a LDMOS transistorand a cavity within a second level of a multilevel metallizationstructure.

FIG. 13 illustrates a plan view of the second level of the multilevelmetallization structure of FIG. 12.

FIG. 14 illustrates a semiconductor device including a LDMOS transistorand a multilevel metallization structure.

FIG. 15 illustrates a semiconductor device including a conductivethrough substrate via.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top”,“bottom”, “front”, “back”, “leading”, “trailing”, etc., is used withreference to the orientation of the figure(s) being described. Becausecomponents of the embodiments can be positioned in a number of differentorientations, the directional terminology is used for purposes ofillustration and is in no way limiting. It is to be understood thatother embodiments may be utilized and structural or logical changes maybe made without departing from the scope of the present invention. Thefollowing detailed description, thereof, is not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims.

A number of exemplary embodiments will be explained below. In this case,identical structural features are identified by identical or similarreference symbols in the figures. In the context of the presentdescription, “lateral” or “lateral direction” should be understood tomean a direction or extent that runs generally parallel to the lateralextent of a semiconductor material or semiconductor carrier. The lateraldirection thus extends generally parallel to these surfaces or sides. Incontrast thereto, the term “vertical” or “vertical direction” isunderstood to mean a direction that runs generally perpendicular tothese surfaces or sides and thus to the lateral direction. The verticaldirection therefore runs in the thickness direction of the semiconductormaterial or semiconductor carrier.

As employed in this specification, when an element such as a layer,region or substrate is referred to as being “on” or extending “onto”another element, it can be directly on or extend directly onto the otherelement or intervening elements may also be present. In contrast, whenan element is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present.

As employed in this specification, when an element is referred to asbeing “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

As used herein, various device types and/or doped semiconductor regionsmay be identified as being of N type or P type, but this is merely forconvenience of description and not intended to be limiting, and suchidentification may be replaced by the more general description of beingof a “first conductivity type” or a “second, opposite conductivity type”where the first type may be either N or P type and the second type thenis either P or N type.

It will be understood by those of skill in the art that the activedevice(s), such as a LDMOS transistor, may be formed on or over thesubstrate or entirely within the substrate or partly within and partlyon or over the substrate, depending upon the nature of the device(s).Accordingly, as used herein with respect to the active device(s), theterms “in the substrate”, “in the semiconductor substrate” andequivalents are intended to include all such variations.

FIG. 1a illustrates a semiconductor device 20 including a semiconductorsubstrate 21 having a front surface 22 and a rear surface 23. Thesemiconductor device 20 also includes an LDMOS transistor 24 in thefront surface 22 and a metallization structure 25 arranged on the frontsurface 22. The metallization structure 25 includes at least one cavity27 arranged in at least one dielectric layer 28.

In the embodiment illustrated in FIG. 1a , the semiconductor substrate21 has a bulk resistivity of greater or equal to 100 Ohm·cm, i.e. ρ≧100Ohm·cm and may include a single crystal substrate, such as a siliconsingle crystal substrate. In this embodiment, the substrate 21 may becalled a highly resistive substrate.

If the bulk resistivity of the semiconductor substrate is equal orgreater than a predetermined level, parasitic inductor-substratecoupling and device parasitics can be substantially reduced. Thedesirable predetermined level is usefully equal or greater than 100Ohm·cm resistivity, conveniently equal or greater than about 500 Ohm·cmresistivity, more conveniently equal or greater than about 1000 Ohm·cmresistivity. As used herein, the term “bulk resistivity” refers to thoseportions of substrate that lie outside the doped device regions of theLDMOS transistor 24.

In other embodiments, such as that illustrated in FIG. 1 b, thesemiconductor substrate 21 includes a layer 21 a epitaxially depositedonto a highly doped substrate 21 b. The highly doped substrate 21 b maybe a highly doped silicon wafer, for example p+, and the epitaxial layer21 a may comprise doped silicon, for example p.

The metallization structure 25 is arranged on the front surface 22 ofthe semiconductor substrate 21 and may include one or more conductivevias 29 which extend through the thickness of the dielectric layer 28 tocouple a conductive layer such as a runner or a contact pad 26 arrangedon the dielectric layer 28 to an electrode of the LDMOS transistor 24.

The cavity 27 may be filled with a material, such as air or a low-kdielectric material or a vacuum, which has a lower dielectric constantthan a dielectric constant of the dielectric material or materialsdefining the cavity, for example the dielectric constant of the materialof the dielectric layer 28. The cavity 27 may be used to provide avolume of localised decreased dielectric constant within themetallization structure 25.

The at least one cavity 27 may be arranged in regions of themetallization structure 25 having an electric field that is greater thanan average electric field within the metallization structure 25. The atleast one cavity 27 may be arranged in regions of the metallizationstructure 25 in order to ensure that a value for the electric field inthat region within the metallization structure 25 remains below athreshold value. For example, the at least one cavity 27 may be arrangedbetween a drain and a gate of the LDMOS transistor in order to reducecapacitive coupling.

As the cavity 27 has a lower dielectric constant than the dielectricmaterial defining it, the cavity may also be used positioned in themetallization structure 25 to reduce capacitive coupling betweenportions of the conductive redistribution structure coupled to differingelectrodes of the LDMOS transistor 24, for example between theconductive redistribution structure coupled to the source and theconductive redistribution structure coupled to the drain.

FIGS. 2a, 2b and 2c illustrate embodiments of the internal structure ofthe dielectric layer 28. The dielectric layer 28 may include two orsublayers which may be arranged to define the cavity 27 in various ways.

In the embodiment illustrated in FIG. 2a , the dielectric layer 28 mayinclude a first sublayer 30 which defines the base and side walls of thecavity 27′ and a second sublayer 31 which is arranged on the firstsublayer 31 and caps and seals the cavity 27′.

In the embodiment illustrated in FIG. 2b , the dielectric layer 28 mayinclude three sublayers, a first sublayer 32, which forms the base ofthe cavity 27′' a second sublayer 33, which is arranged on the firstsublayer 32 and forms the side walls of the cavity 27″ and a thirdsublayer 34 which is arranged on the second sublayer 33 and caps andseals the cavity 27″.

In the embodiment illustrated in FIG. 2c , the dielectric layer 28includes four sublayers. A first sublayer 35 forms the base of thecavity 27′″, a second and a third sublayer 36, 37 form the sidewalls ofthe cavity 27′″ and a fourth sub-layer 36 forms the top or cap of thecavity 27′″.

The sublayers 30 to 38 of the dielectric layer may include the samedielectric material or differing dielectric materials. For example,alternate layers may include an oxide, such as SiO, or SiO₂ and theintervening layer includes a nitride such as SiN_(x).

In some embodiments, the cavity 27 is defined on all sides by one ormore dielectric materials. In some embodiments, one or both of the topand base of the cavity 27 may be provided by a semiconductor layer or ametallic layer.

The structure of the LDMOS transistor 24 is illustrated in more detailin FIG. 3, whereby FIG. 3a illustrates a cross-sectional view and FIG.3b illustrates a top view of a portion of the LDMOS transistor 24. Thesemiconductor substrate 21 may be a highly resistive substrate asillustrated in FIG. 1a or include an epitaxial layer 21 a arranged on ahighly doped substrate 21 b as illustrated in FIG. 1 b.

The LDMOS transistor 24 may include a plurality of transistor cells 40each including a doped source region 41, a doped drain region 42 and agate 43 arranged in the front surface 22 of the semiconductor substrate21. The gate 43 may be laterally arranged between the source region 41and drain region 42. The gate 43 may be asymmetrically arranged betweensource region 41 and drain region 42 such that the distance between thesource region 41 and the gate 43 is less than the distance between thegate 43 and the drain region 42.

The LDMOS transistor 24 includes a plurality of transistor cells 40 inwhich the arrangement of source region 41 and gate 43 of neighbouringtransistor cells 40 has a symmetrical arrangement around a common drainregion 42 such that the pattern of source gate drain gate source gatedrain gate source is formed. The source region 41, the gate 43 and thedrain region 42 have an elongate striped form as can be seen in the planview of FIG. 3 b.

The metallization structure 25 may include separate conductiveredistribution paths to electrically couple the source regions 41 of thetransistors cells 40 to a common source contact pad, the drain regions42 of the transistor cells 40 to a common drain pad and the gates 43 ofthe transistor cells 40 to a common gate pad.

The LDMOS transistor 24 includes a source region 41 which is highlydoped with a second conductivity type, for example n+, and a drainregion 42 which is highly doped with the second conductivity type, forexample n+. The source region 41 and the drain region 42 are arranged atthe front surface 22 of the semiconductor substrate 21 asymmetricallyaround the gate 43 arranged on the front surface 22.

A drift region 44 extends into the front surface 22 under the gate 43 inthe direction of the drain region 42 and may be doped with the secondconductivity type, for example n. The drift region 44 extends furtherinto the semiconductor substrate 11 than the drain region 42. The driftregion 44 is doped with the second conductivity type, for example n, andhas a dopant concentration which is less than the dopant concentrationof the drain region 42 and of the source region 41.

The LDMOS transistor 24 further includes a channel region 45 doped withthe first conductivity type which extends from the drift region 44 underthe source region 41 and a body contact region 46 which is doped withthe first conductivity type and extends from the front surface 22 intothe semiconductor substrate 21 deeper than the channel region 45. Thedrift region 44 extends into the semiconductor substrate 11 by a greaterdistance than a channel region 45. The body contact region 46 isprovided by a highly doped well of the second conductivity type, forexample p+.

In some embodiments, the semiconductor substrate 21 may include siliconwhich is lightly doped with a first conductivity type, for example p−.In embodiments in which the semiconductor substrate 21 includes a highlydoped substrate 21 b and epitaxial layer 21 a, the highly dopedsubstrate 21 b may be p+ and the epitaxial layer p-doped.

When the gate 43 is appropriately biased, a conductive channel formsbetween the source region 41 and the drain region 42. The exemplaryconductivity types presented above are suitable for forming an N-channelstructure, but a P-channel structure can also be formed by appropriateinterchange of conductivity type of the various doped regions andappropriate modification of the bias on gate 43.

The source region 41 of the LDMOS transistor 24 may be coupled to therear surface 23 of the semiconductor substrate 21. The conductive pathbetween the source region 41 and the rear surface 23 may have differentforms.

In some embodiments, a conductive via is provided which extends from thefront surface 22 to the rear surface 23 through the semiconductorsubstrate 21. The conductive via may be positioned such that the bodycontact 46 surrounds the upper portion of the conductive via. Theconductive via may be lined with a metal, such as tungsten or copper. Insome embodiments, the base of the conductive via is filled with highpurity copper and the upper portion of the via includes high puritycopper clad sidewalls which surround a gap. The top of the via may besealed to provide a cavity or void within the upper portion of thethrough substrate via. A conductive path may also be provided from thefront surface 22 to the rear surface 23 by a highly doped sinkerstructure.

One or more cavities 27 may be positioned adjacent each side of thecommon drain 42 of two adjacent transistor cells 40. A plurality ofcavities 27 may be arranged in a row adjacent each side of the drain 42as can be seen in the plan view of FIG. 3b . Each cavity 27 may have anelongate structure as the source region 41, gate 43 and drain region 42of the LDMOS transistor cells typically have the form of elongatestripes. However, the lateral shape of the cavities 27 is not limited toa rectangular lateral shape and may have an oval shape or an elongateshape with rounded ends. The lateral shape of the cavity 27 is also notlimited to an elongate shape and may be circular, square, or hexagonal,for example. The length of the cavities 27 and the distance betweenneighbouring cavities may be selected such that the dielectric layer 28has suitable mechanical properties.

A semiconductor device including a substrate, a LDMOS transistor and ametallization structure may include a multilevel metallization structureincluding two or more dielectric layers and intervening conductivelayers. In embodiments, in which the metallization structure has amultilevel structure, at least one cavity may be arranged in one, two,some or all of the dielectric layers.

FIG. 4 illustrates a semiconductor device 50 according to an embodimentwhich includes a substrate 51 with a front surface 52 and rear surface53, a LDMOS transistor 54 arranged in the front surface 52 and amultilevel metallization structure 55 arranged on the front surface. Thesubstrate 51 may include a semiconductor, such as a silicon wafer, andhave a bulk resistivity of greater or equal to 100 Ohm·cm, or include anepitaxial layer on a highly doped substrate.

A first metallization level 56 may include a first dielectric layer 57arranged on the front surface 52 of the substrate 51 and a first metallayer 58 arranged on the first dielectric layer 57. A secondmetallization level 59 may include a second dielectric layer 60 arrangedon the first metal layer 58 and a second metal layer 61 arranged on thesecond dielectric layer 60. The metallization structure 55 may alsoinclude a third dielectric layer 62 arranged on the second metal layer61 and a third metal layer 63 arranged on the third dielectric layer 62.The third metal layer 63 may provide one or more contact pads 64 of thesemiconductor device 50.

As an example, a drain 65 of two neighbouring transistor cells 66, 66′of the LDMOS transistor 54 may be electrically coupled to the contactpad 64 by a first conductive via 67 which extends through the firstdielectric layer 57, a portion of the first metal layer 58, whichprovides a drain runner 68 and is arranged on the first conductive via67, a second conductive via 69 which extends through the seconddielectric layer 60 and is positioned on the drain runner 68 and extendsinto a portion providing a further drain runner 80 in the second metallayer 61 and a third conductive via 70 which extends through the thirddielectric layer 62 is arranged on the further drain runner 80. Theconductive pad 64 may be arranged on the third conductive via 70. Thefirst, second and third conductive vias 67, 69, 70 and the drain runner68 of the first metal layer 68 and further drain runner 80 of the secondmetal layer 61 may be arranged in a stack.

The first conductive layer 58 and the second metal layer 61 includeconductive portions 68, 80 arranged in a dielectric material 79. Theconductive portions 68, 80 may have lateral extent which is greater thanthe lateral extent of the underlying conductive via 67, 69.

The metallization structure 55 may include a first cavity 71 arranged inthe first dielectric layer 57 adjacent a first side 72 of the firstconductive via 67 and a second cavity arranged 73 in the seconddielectric layer 60 adjacent a first side 74 of the second conductivevia 69. The second cavity 73 may extend through the second dielectriclayer 60 and the dielectric material 79 of the second metal layer 61.

In embodiments in which the transistor cells 66, 66′ of the LDMOStransistor 54 are arranged symmetrically about the drain 65, themetallization structure 55 may include a third cavity 75 arranged infirst dielectric layer 57 adjacent a second side 76 of first conductivevia 67, the second side 76 opposing the first side 72, and a fourthcavity 77 extending through the second dielectric layer 60 and secondmetal layer 61 and adjacent a second side 78 of the second conductivevia 69, the second side 78 opposing the first side 73.

The lateral extent of the conductive portions providing the drainrunners 68, 80 may determine the minimum distance between the firstcavity 71 and the third cavity 75 and the first conductive via 67 andbetween the second cavity 73 and fourth cavity 77 and the secondconductive via 69. In the embodiment illustrated in FIG. 4, thedielectric material 79 of first conductive layer 58 may be used to capand seal the first cavity 71. The dielectric material 79 of the thirddielectric layer 62 may be used to cap and seal the second cavity 73 andthe dielectric material 79 of the first metal layer 58 may be used toprovide the base of the second cavity 73.

The conductive vias 67, 69, 70 and the conductive layers 58, 61 mayinclude one or more metals. In one embodiment, the first conductive via67 includes tungsten and the first and second conductive layers 58, 61and second and third conductive vias 69, 70 may include high puritycopper.

In some embodiments, the sidewalls and base of the conductive vias 67,69, 70 may include one or more further layers which may provide adhesionpromotion function between the tungsten or copper of the conductive viaand the sidewalls of the via formed by the dielectric material. Forexample, a Ti, TiN Cu seed layer, Cu stack may be arranged on thesidewalls of the conductive vias 69, 70. The contact pad 64 may includecopper and may include an outermost conductive layer which is easilysolderable. In some embodiments, the contact pad 64 includes anoutermost gold layer or aluminium layer.

The dielectric layers 57, 60, 62 and the dielectric material 79 of theconductive layers 58, 60 may include differing materials. In someembodiments, the dielectric layers 57, 60, 62 include SiO or SiO₂ andthe dielectric material 79 of the conductive layers 58, 60 includes anitride such as SiNx.

The cavities 71, 75, 73, 77 may be positioned within the metallizationlevel 56, 59 in regions of high electric field strength to effectivelyreduce capacitive coupling of the different electrode structuresinvolved.

For a metallization structure 55 including high purity copper vias andcopper conductive layers, damascene techniques may be used. In someembodiments, the copper portions of the first conductive layer 58 may beformed by a single damascene technique and the second conductive via 69and further drain runner 80 may be formed using a dual damascenetechnique such that the second conductive via 69 and the further drainrunner 80 are formed in a single deposition process and are integral.

FIG. 5a illustrates a schematic flow diagram 90 of a method formanufacturing a semiconductor device. In block 91, at least onedielectric layer is applied to a front surface of a semiconductorsubstrate including a LDMOS transistor structure. In block 92, anopening is formed in at least one dielectric layer. In block 93, theopening is covered with a further dielectric layer to seal the cavity.

The cavity maybe filled with a material which has a lower dielectricconstant than that of the dielectric layer defining the walls of thecavity. For example, the cavity may include air or a vacuum. Therefore,the cavity may be used to provide a localised reduction in thedielectric constant of the dielectric layer and to reduce parasiticcoupling by suitable placement within the metallization structure.

FIG. 5b illustrates a flow diagram 100 of a method for forming theopening. In this embodiment, in block 101, a first opening is formed inat least one dielectric layer and filled with a sacrificial material. Inblock 102, a second dielectric layer is formed on the first dielectriclayer. In block 103, at least one second opening is formed in the seconddielectric layer to expose the region of the conductive material withinthe first opening. In block 104, the sacrificial material is removedfrom the first opening through the second opening. In block 105, afurther layer is applied to the second dielectric layer to close andseal the second opening and thus seal the cavity. The further layer maybe a dielectric material, thus forming a cavity defined on all sides bydielectric material.

The sacrificial material may be a conductive material and may be thesame conductive material as that inserted into the conductive viascoupled to one or more doped regions of the LDMOS transistor.

In one embodiment, the method may be carried out at the same time as theformation of the conductive vias to one or more doped regions of theLDMOS transistor. In this embodiment, the conductive material arrangedin the conductive vias coupled to an electrode of the LDMOS transistorremains covered and protected by the second dielectric layer whilst thesacrificial material is removed through the second openings from thefirst openings filled with conductive material that is not coupled to anelectrode of the LDMOS transistor. The sacrificial material-filled firstopenings may be termed dummy structures or dummy conductive vias as theydo not form part of the conductive redistribution structure of themetallization structure.

By fabricating the cavities by first filling the openings in the firstdielectric layer with the conductive material inserted into theconductive vias, the number of additional steps required may beminimised, since the additional steps in comparison with the fabricationof the conductive via are limited to the formation of openings in thethird dielectric layer and the etching process to remove the conductivematerial.

The position of the cavities may be selected so as to reduce capacitivecoupling between one or more of the conductive paths provided by themetallization structure for the electrodes of the LDMOS transistor, forexample to reduce capacitive coupling between the gate and drainelectrodes, between the gate and source electrodes, and/or between thesource and drain electrodes in embodiments in which the metallizationstructure is multilevel metallization structure, one or more cavitiesmay be positioned in one or more layers of the multilayer metallizationstructure.

The openings may be formed in the first dielectric layer and in thesecond dielectric layer by applying a structured mask. The openings inthe second dielectric layer may be laterally smaller than the lateralarea of the first opening. In some embodiments, two or more discreteopenings are formed above a single conductive via which acts as aprecursor for a cavity or as a dummy structure. The number, size andarrangement of the openings in the second dielectric layer may beselected so as to allow complete etching of the conductive materialwithin the second conductive via so that the cavity is bounded bydielectric material only and such that the opening can be reliablycovered by the application of further dielectric layer so as to form anenclosed and sealed cavity. The cavity may extend onto one or morefurther dielectric layers such that the sidewalls of the cavity areformed by two or more stacked dielectric layers.

The conductive vias coupled to the LDMOS transistor and the conductivevias acting as dummy structures may further include one or moreadditional layers arranged on the sidewalls and optionally the base ofthe opening. The one or more additional layers may include Ti, and/orTiN for a tungsten filled via or Ta and/or TaN for a Cu filled via andserve as adhesion promoters and/or diffusion barriers.

The conductive material including any additional liner layers may beremoved from the second conductive via by etching techniques, includingreactive ion etching and wet etching or a combination of etchingtechniques.

FIG. 6 illustrates a cross-sectional view of a semiconductor device 110including a substrate 111 and a LDMOS transistor 112 formed in a front,surface 113 of the substrate 111. The substrate 111 may include asilicon single crystal and has a bulk resistivity of at least 100Ohm·cm.

The LDMOS transistor 112 includes a highly doped source region 114, ahighly doped drain region 115 and a gate 116 arranged laterally betweenthe highly doped source region 114 and highly doped drain region 115.The LDMOS transistor 112 also includes a doped drift region 117 whichextends from the front surface 113 into the substrate 111 from the gate116 to the highly doped drain region 115, a doped channel region 118which extends from the front surface 113 into the substrate 111 and fromgate to the highly doped source region 114 and a doped body contactregion 119. The doped body contact region 119 extends further into thesubstrate 111 from the front surface 113 than the doped channel region118.

The highly doped source region 114, the highly doped drain region 115and the drift zone 117 have a first conductivity type, for examplen-type, which opposes the conductivity type of the substrate 111. Thechannel region 118 and the body contact region 111 have a secondconductivity, for example p-type, which opposes the first conductivitytype.

The LDMOS transistor 114 also includes a field plate 120 which extendsfrom the gate 116 in the direction of the highly doped drain region 115and a gate shield 121 which extends from the gate 116 in the directionof the highly doped source region 114. The LDMOS transistor 114 alsoincludes a doped buried layer 147 coupled to the body contact region 111which extends throughout the lateral area of the substrate. The dopedburied layer 147 is spaced apart and at a distance from the frontsurface 113 and rear surface of the substrate 111 and along with thefield plate 120 and drift region 117 forms part of a RESURF structurefor the LDMOS transistor 114.

The semiconductor device 110 includes a multilevel metallizationstructure arranged on the front surface 113 which provides a conductingpath from the highly doped source region 114, highly doped drain region115 and gate 116 to a respective contact pad at the outer surface of thesemiconductor device 110.

The first level of metallization 122 includes a stack of four dielectriclayer including a SiP₂ layer 123 arranged on the front surface 113 aBPSG layer 124 arranged on the SiO₂ layer 123, a silicon nitride layer125 arranged on the BPSG layer 124, a SiO_(x) layer 126 arranged onsilicon nitride layer 125 and a further silicon nitride layer 127arranged on the SiO_(x) layer 126.

Vias are formed through this multilayer dielectric stack to provide aconductive via 128 arranged on and coupled to the highly doped drainregion 115, a conductive via 129 which is coupled to the gate and aconductive via 130 which is coupled to the highly doped source region114. The electrical contact between the conductive via 129 and the gate116 takes place outside of the plane illustrated in FIG. 6.

Two further conductive vias 131, 132 are positioned adjacent opposingside faces 133 134 of the conductive via 128 which is coupled to thehighly doped drain region 115. The conductive vias 128, 129, 130, 131,132 may include one or more liners such as a Ti layer and TiN layerpositioned on the sidewalls and the base. The conductive vias 128, 129,130, 131, 132 are filled with a conductive material which, in thisembodiment, is tungsten. The upper surface 135 may be planarised suchthat the upper surface of the conductive material of the vias 128, 129,130, 131, 132 and the SiO_(x) layer 126 is substantially coplanar.

The conductive vias 128, 129, 130, 131, 132 are in each case one of arow of vias which extends into the plane of the drawing.

The conductive vias 131, 132 are now subjected to additional processesto form cavities within the dielectric material of the firstmetallization level 122. The conductive vias 131, 132 may be consideredas dummy structures since they do not form part of the conductiveredistribution structure of the metallization structure.

As is illustrated in FIG. 7, a silicon nitride layer 136 is applied tothe SiOx layer 126 and covers the conductive vias 128, 129, 130. Aresist mask 139 is applied to the nitride layer 136 and structured so asto form openings 137, 138 above the conductive vias 131, 132. Theopenings 137, 138 are laterally smaller than the lateral area of theconductive vias 131, 132 at the surface 135.

As is illustrated in FIG. 8, the conductive material including the linerlayers is removed from the conductive vias 131, 132, for example byusing etching techniques, to produce two openings 140, 140′ through thedielectric stack adjacent to, and spaced apart from, the conductive via128 coupled to the highly-doped drain region 115.

As is illustrated in FIG. 9, a dielectric layer 141, for exampleincluding SiO₂, is applied to the silicon nitride layer 136 which coversthe openings 137, 138 in the silicon nitride layer 136 and formscavities 144, 145 within the dielectric material of the firstmetallization level 122.

The SiO₂ layer 141 and the silicon nitride layer 136 is patterned toform openings to the conductive vias 128, 130 which are coupled to thehighly doped drain region 115 and highly doped source region 114,respectively.

High purity copper is deposited and fills the openings and is thereforeelectrically coupled to the conductive material within the vias 128,130. A planarisation process is carried out to produce discrete copperportions arranged within the thermal oxide layer 141. Such a process maybe denoted as a single Damascene process.

Before deposition of the high purity copper, a tantalum nitride,tantalum, copper seed layer stack may be deposited within the openingsand the copper material which fills the openings may be deposited usingan electroplating technique. The surface may then be planarised, forexample using chemical mechanical polishing (CMP).

FIG. 10 illustrates a plan view of the arrangement of the cavities 144,145 and the conductive connection to the highly doped drain region 115.

As can be seen in the plan view, the highly doped drain region 115 andthe drain runner 146 have an elongate, strip-like form. The drain runner146 is laterally slightly wider than the highly doped drain region 115.

A plurality of conductive vias 128 is provided, each of which has alateral area which is less than the lateral area of the drain runner146. The conductive vias 128 are arranged at intervals along the lengthof the highly doped drain region 115 and drain runner 146. The drainrunner 146 is arranged on and electrically coupled with the plurality ofcontact vias 128.

A plurality of cavities 144, 144′ is arranged in a row adjacent andsubstantially parallel to the first side 150 of the drain runner 146 andthe conductive vias 128 and a plurality of cavities 145, 145′ isarranged adjacent the opposing side 151 of the drain runner 146. As canalso be seen in the plan view, the openings 143 to the cavities 144,144′ and 145, 145′ are laterally smaller than the lateral area of thecavity 144, 144′ and 145, 145′. The minimum distance between thecavities 144, 145 and the respective side of the conductive via 128 isdetermined by the lateral width of the drain runner 146 as the lateralwidth of the drain runner 146 is greater than the lateral width of theconductive vias 128.

FIG. 11 illustrates the semiconductor device 110 after deposition of asecond metallization level 160 arranged on the first metallization level122. The second metallization level 160 may include a stack ofdielectric layers including a silicon nitride layer 161 arranged on theSiO₂ layer 141, a SiO₂ layer 162 arranged on the silicon nitride layer161, silicon nitride layer 163 arranged on the SiO₂ layer 162, a SiO₂layer 164 arranged on the silicon nitride layer 163, a silicon nitridelayer 165 arranged on the SiO₂ layer 164 and a SiO₂ layer 166 arrangedon the silicon nitride layer 165.

The second metallization level 160 includes metal via portions 167 whichextend through the silicon nitride layer 161 and SiO₂ layer 162. Thesecond metallization level 160 further includes a conductiveredistribution structure which extends through the silicon nitride layer163 and SiO₂ layer 164. In the case of the metallization structure 169for the drain contact, a plurality of conductive vias 167 may beprovided which are arranged spaced at intervals along the length of theunderlying drain runner 146. The redistribution portion 168 may haveelongate substantially strip-like form. The conductive via 167 andconductive portion 168 may be fabricated by forming openings in thestack of dielectric layers and filling the openings using a dualDamascene technique to form both the conductive vias in the layers 161,162 and the conductive portion 166 in the dielectric layers 163, 164using a single deposition process.

In embodiments in which cavities are formed in the second metallizationlevel 160, such as that illustrated in FIG. 12, the cavities may beformed by forming sacrificial metal portions or dummy structures 170within the stack of dielectric layers 161, 162, 163, 164 along with theconductive vias 167 and redistribution layer 166. The dummy structures170 may have an elongate shape and be arranged in a row extendingsubstantially parallel to side face of the conductive structure of thedrain contact stack 169.

In some embodiments, the drain sided edge of one row of the dummystructures 170 may be positioned above the gate.

In order to remove the copper from the dummy structures 170, openingsmay be formed in the silicon nitride layer 165 and SiO₂ layer 166covering the top of the conductive layer 168. One or more openings 171to each of the dummy structures 170 may be formed in the silicon nitridelayer 165. The conductive material, in particular high purity copper ofthe dummy structure 170, may be removed through these openings 171 usingetching processes to form a cavity 172 which is defined by dielectricmaterial.

As is illustrated in FIG. 12, the openings 171 in the silicon nitridelayer 165 may be covered by subsequently depositing a further oxidelayer 173 to form a sealed cavity 172. One or more further cavities mayalso be formed on the opposing side of the drain contact structure 169.

FIG. 13 illustrates a plan view of the second metallization level 160and illustrates the strip-like drain runner 168 arranged on andelectrically coupled with a plurality of conductive vias 167. Thecavities 172 and the plurality of openings 171 are arranged in a rowadjacent to opposing long sides of the drain runner 168 and partiallyoverlap with the gate fingers of the underlying metallic layer. A sourcerunner is arranged adjacent each row of cavities and gate runneradjacent the opposing side of the source one.

FIG. 14 illustrates a further metallization level 180 to provide aconductive structure between the drain metal stack 169 and an outercontact pad 181. The further metallization level 180 may include one ormore dielectric layers 182, 183, a metal contact 184 extending throughthe dielectric layers 165 and 166 and one or more through contacts 185.The contact pad 181 may also be defined on the upper surface bydepositing a further nitride layer 186 which may overlap some or allperipheral regions of the metal contact pad 181.

In some embodiments, such as that illustrated in FIG. 14, cavities 144,145, 172 are arranged within differing levels 122, 160 of the multilevelmetallization structure 155. However, cavities may be arranged in onlyone of the metallization layers or more than one of the metallizationlayers. The position of the cavities may be selected such that alocalised region of the dielectric material of the metallizationstructure 155 is provided with a lower dielectric constant at positionswithin the metallization structure suitable for reducing the maximumelectric field at that position and for reducing capacitive couplingbetween two or more separate conductive portions of the redistribution.structures provided by the metallization structure, for example, betweenthe source distribution structure and the drain redistributionstructure.

The highly doped source region 114 of the LDMOS transistor 112 may beelectrically coupled to a rear surface of the substrate 11 by one ormore conductive through substrate vias 190 arranged adjacent the LDMOStransistor 112 and, in some embodiments, between highly doped sourceregions 114 of neighbouring LDMOS transistor cells or segments. Theright-hand side of a portion of one of a row of conductive throughsubstrate vias 190, which extends into the plane of the drawing, isillustrated in FIGS. 6 to 14.

FIG. 15 illustrates an alternative view of the semiconductor device 110and the conductive through substrate via 190. The conductive throughsubstrate via 190 may be fabricated after the fabrication of the LDMOStransistor 112 and before the metallization structure 120 is applied tothe front surface.

The conductive through substrate via 190 includes a via 191 formed thesemiconductor substrate 111 which extends from the front surface 113 tothe rear surface 192 and has side walls 193 defined by the material ofthe semiconductor substrate 111.

A multilayer lining 194 is deposited on the side walls 193 of the blindvia 100. The multilayer lining 194 may include adhesion and/or barrierlayers, for example. Conductive material 195 including high puritycopper is deposited on the multilayer lining 101 in the via 191 andfurther extends over the front surface 113 of the substrate 111 and iscoupled to the doped source region 114 of the LDMOS transistor 112.

The conductive material 195 may be deposited using a two stage processand includes two subparts. A first subpart 196 is arranged in the baseof the via 191 which completely fills the base of the via 191 to providea type of conductive plug arrangement at the rear surface 192 of thesubstrate 111 The second subpart 197 has the form of a layer whichcovers the side walls 193 of the via 191 and the upper surface 198 ofthe conductive plug 196 such that the second subpart 197 surrounds a gap199 in the upper portion of the via 191. The second subpart 197 may havea U-shaped form the as-deposited state and an open-ended ring shapedform in an annealed state.

The first subpart 196 and the second subpart 197 may be deposited byelectroplating. The conditions used to electroplate the first subpart196 and the second subpart 197 may be selected in order that verticalgrowth is favoured and lateral growth is suppressed during deposition ofthe first subpart 196 in order to fill the volume of the via 191 at thebase and so that lateral growth is favoured during growth of the secondsubpart 196 so as to produce a lining which sounds the gap 199. Thefirst subpart 196 and the second subpart 197 may have differentmicrostructures. For example, the first subpart 196 may have an averagegrain size that is larger than an average grain size of the secondsubpart 197.

In subsequent processing steps the substrate 111 may be subjected toraised temperatures, for example during a subsequent annealing process.The raised temperatures may cause grain growth within the first subpart196 and second subpart 197 such that the interface between the base ofthe second subpart 196 and first subpart 197 is no longer discernible.The region of the second subpart 197 arranged on the side walls 193 ofthe via 191 may be discernible from the first subpart 196 which fillsthe via 191 by a difference in microstructure, for example a smalleraverage grain size.

The conductive portions 200 of the second subpart 197 arranged on thefront surface 113 may be substantially surrounded by dielectricmaterial. Such arrangements and methods are known as dual damasceneprocesses, since a vertical conductive layer is formed in the via 191and a lateral conductive layer is formed on the front surface 113 of thesubstrate 111 using a single deposition process and conductive portionsembedded in a dielectric matrix are formed.

A cavity 201 is arranged in the upper portion of the conductive throughsubstrate via 191. The cavity 201 may be defined by dielectric material.In some embodiments, a first dielectric material 202 lines the gap 199formed in the via 191 by the second subpart 197 and the first subpart196 of the conductive material 195. A second dielectric material 203caps the gap 199 and together with the first dielectric material 202defines a sealed and enclosed cavity 201.

The conductive through substrate via 190 is partially filled andincludes a dielectric defined cavity 201 in its upper portion. Theconductive through substrate via 190 includes a bottom closed metalplane provided by the conductive plug 196 and a top plane which isformed by the dielectric layer 203 sealing the cavity 199. The closedmetal plane at the bottom prevents contamination of the via duringworking of the rear surface 192 of the substrate 111 and the cavity 201provides an expansion volume.

The conductive through substrate via 190 may be fabricated by insertingthe conductive material 195 into a blind via and working the rearsurface of the initial substrate to remove material and expose the base204 of the first subpart 196 in the rear surface 192 of the finalsubstrate 111 in order to electrically couple the source 114 of theLDMOS transistor 112 to the rear surface 191 of the substrate 11. Thefinal thickness of the substrate 111, t_(f), may be around 60 μm.

One or further conductive layers 205 are arranged on the rear surface191 of the semiconductor substrate 111 and base 204 of the conductivevia 191. The conductive layer 205 is coupled to the source 114 by meansof the conductive material of the first subpart 196 and second subpart197. The conductive layer 205 on the rear surface 191 of the substrate111 may be used to mount the device and electrically couple the source114 to a die pad or flange.

The semiconductor device including one or more LDMOS transistorsaccording to any one of the embodiments described herein may be used ina high frequency power amplifying circuit, such as RF power amplifyingcircuits for use in cellular communications operating at frequencies inthe range of 700 MHz to 3.6 GHz, power conversion in cellularcommunication networks and Doherty configuration amplifying circuits.

Spatially relative terms such as “under”, “below”, “lower”, “over”,“upper” and the like are used for ease of description to explain thepositioning of one element relative to a second element. These terms areintended to encompass different orientations of the device in additionto different orientations than those depicted in the figures. Further,terms such as “first”, “second”, and the like, are also used to describevarious elements, regions, sections, etc. and are also not intended tobe limiting. Like terms refer to like elements throughout thedescription.

As used herein, the terms “having”, “containing”, “including”,“comprising” and the like are open ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a”, “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise. It is to be understood that the features of thevarious embodiments described herein may be combined with each other,unless specifically noted otherwise.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. A semiconductor device, comprising: asemiconductor substrate comprising a front surface, a LDMOS (LateralDiffused Metal Oxide Semiconductor) transistor in the front surface; anda metallization structure arranged on the front surface, wherein themetallization structure comprises at least one cavity arranged in atleast one dielectric layer.
 2. The semiconductor device of claim 1,wherein the at least one cavity is defined by dielectric material on allsides.
 3. The semiconductor device of claim 1, wherein the at least onecavity is arranged in regions of the metallization structure having anelectric field that is greater than an average electric field of thesemiconductor device.
 4. The semiconductor device of claim 1, whereinthe at least one cavity is arranged between a drain region and a gate ofthe LDMOS transistor so as to reduce capacitive coupling.
 5. Thesemiconductor device of claim 1, wherein the metallization structurecomprises a first dielectric layer arranged on the front surface, afirst conductive layer arranged on the first dielectric layer and atleast one first conductive via, wherein the at least one cavity isarranged adjacent a side face of the at least one first conductive via.6. The semiconductor device of claim 5, wherein the at least one firstconductive via electrically couples a drain of the LDMOS transistor tothe first conductive layer.
 7. The semiconductor device of claim 5,wherein a first cavity is arranged adjacent a first side of the at leastone first conductive via and a second cavity is arranged adjacent asecond side of the at least one first conductive via, the second sideopposing the first side.
 8. The semiconductor device of claim 5, whereinthe first cavity extends through the first dielectric layer and isbounded on a lower surface by a second dielectric layer and on an uppersurface by a third dielectric layer, wherein the third dielectric layercomprises a plurality of openings in communication with the firstcavity, the plurality of openings being covered by a fourth dielectriclayer.
 9. The semiconductor device of clam 8, wherein the firstdielectric layer comprises three sublayers, the first sublayercomprising BPSG, the second sublayer comprising SiN and the thirdsublayer comprising SiOx, and wherein the second dielectric layercomprises SiON, the third dielectric layer comprises SiN and the fourthdielectric layer comprises SiOx.
 10. The semiconductor device of clam 5,wherein the first cavity is arranged between drain-sided edge of a fieldplate and a drain region of the LDMOS transistor.
 11. The semiconductordevice of claim 8, further comprising at least one third cavity in fifthdielectric layer arranged on the fourth dielectric layer.
 12. Thesemiconductor device of claim 11, further comprising a second conductivevia extending through the fifth dielectric layer, wherein the at leastone third cavity is arranged adjacent a side face of the secondconductive via.
 13. The semiconductor device of claim 12, wherein thesecond conductive via is integral with a runner arranged on the fifthdielectric layer.
 14. The semiconductor device of claim 1, wherein afirst plurality of cavities is arranged in a first row adjacent a firstside face of a conductive via extending though the first dielectriclayer.
 15. The semiconductor device of claim 14, wherein a secondplurality of cavities is arranged in a second row adjacent a second sideface of the conductive via extending though the first dielectric layer,the second side face opposing the first side face.
 16. The semiconductordevice of claim 1, wherein the semiconductor substrate has a bulkresistivity ρ≧100 Ohm·cm.
 17. The semiconductor device of claim 16,further comprising a conductive via extending from the front surface toa rear surface of the semiconductor substrate, the conductive via beingcoupled to a source of the LDMOS transistor.
 18. An LDMOS transistor,comprising: a semiconductor substrate comprising a front surface, adoped source region, a gate, a doped drain region, a doped channelregion, a doped drain drift region and a doped body contact regionarranged in the front surface; a metallization structure arranged on thefront surface, the metallization structure comprising one or moredielectric layers, one or more conductive redistribution layers arrangedbetween the one or more dielectric layers and one or more conductivevias extending through the one or more dielectric layers; and at leastone cavity defined by dielectric material, the at least one cavitycomprising a dielectric constant which is lower than a dielectricconstant of the dielectric material defining the at least one cavity.19. The LDMOS transistor of claim 18, wherein the at least one cavity isarranged adjacent opposing sides of a conductive drain runner.
 20. TheLDMOS transistor of claim 18, wherein the at least one first cavity isarranged in a first dielectric layer and at least one second cavity isarranged in a second dielectric layer such that the cavities have anoff-set stacked arrangement within the metallization structure.
 21. TheLDMOS transistor of claim 18, wherein a lowermost conductive viaarranged on a transistor contact comprises tungsten and a furtherconductive via coupled to the first conductive via comprises high puritycopper.
 22. The LDMOS transistor of claim 18, further comprising athrough substrate via, extending from the front surface to a rearsurface of the semiconductor substrate.
 23. The LDMOS transistor ofclaim 22, wherein the through substrate via comprises a first portionadjacent the rear surface that fills the via and a second portion on thefirst portion that lines side faces of the via and surrounds a gap. 24.A method, comprising: applying a dielectric layer to a front surface ofa semiconductor substrate comprising a LDMOS transistor structure;forming an opening in at least one dielectric layer; and covering theopening with a further dielectric layer to seal the cavity.
 25. Themethod of claim 24, wherein forming the opening comprises: forming afirst opening in the first dielectric layer and filling the firstopening with a sacrificial material; forming a second dielectric layerover the first dielectric layer; forming a second opening in the seconddielectric layer to expose a region of the sacrificial material withinthe first opening; removing the sacrificial material from the firstopening through the second opening; and applying the further dielectriclayer on the second dielectric layer to close and seal the secondopening and seal the cavity.
 26. The method of claim 25, wherein thesecond dielectric layer covers a conductive via during removal of theconductive material from the first opening.
 27. A method, comprising:positioning one or more cavities having a dielectric constant that isless than a dielectric constant of dielectric material of ametallization structure arranged on a LDMOS transistor in positions soas to reduce one or more of the group consisting of an electric field inthe metallization structure at those positions, capacitive couplingbetween drain and gate, capacitive coupling between source and gate, andcapacitive coupling between source and drain.